1. Field of the Invention
The invention relates to a fault current protection switch with windings of at least one difference transformer connected into each path of the supply mains. The transformer operates by difference measurement to detect any fault current to ground and switching off the supply mains via control and switching apparatus.
2. Description of the Prior Art
Generally speaking, in currently available electrical equipment any exposed metal parts are grounded in order to avoid the occurrence of voltages therein which may result through insulation damage and which may be dangerous to bodily contact. Since the protective ground connections can be broken by error, fault current protection switches are provided. When fault currents occur of a given magnitude to ground, these switches disconnect the supply voltage. They are designed to be sensitive and respond rapidly to ensure that when a faulty device is grounded via a human being no currents dangerous to the human body can occur.
Previously known fault current protection switches operate with a difference transformer in which all the measuring windings for the various conductors of the supply mains are connected such that their currents normally cancel one another out. As a result, a trip coil of the fault current protection switch normally has no current flow therethrough and does not allow the protection switch to respond. Fault currents, on the other hand, disturb this equilibrium in the measuring windings and induce a voltage in the trip winding and thus cause the protection switch to respond. In the present state of the art high-grade transformers can possibly be used to reduce the trip current to approximately 30 milliamperes, although this does not entirely rule out the danger to humans as a result of bodily contact currents.
In addition, even in these high-grade transformers their transmission properties are impaired by a d.c. component of the fault current. For such currents they become less sensitive or, in the case of a very high d.c. component, they may no longer respond at all. More recent developments in electrical technology have resulted in an increasing use, even in normal electrical devices, of semiconductors which can draw fault currents with high d.c. components. In all these situations it is almost impossible to use the previously known fault current protection switches.